Spring rates & damper lengths

NOTE: The front spring rates of my car are not applicable to the Mojo SE and Riot as they use a different inboard front suspension design which gives a much more direct lever ratio, hence these variants need to run a significantly softer spring to give the same wheel rate.

My 'day job' involves analysing different suspension designs using CAE.  I use 2 pieces of software (Adams Chassis and Simpack) to evaluate the performance of a suspension design, and then change the various tunable items in the suspension to improve the performance based on various targets.  The tunable items include obvious things such as spring rates and damper tune, but also tyres, bush rates, the hard points of the suspension, bump stops, spring aids, rebound stops... the list goes on.

So, it was fairly inevitable that I would want to start messing around with the suspension of the Mojo!

Damper lengths

One thing that I noticed from way back when I setup the ride heights was that the Mojo lacked rebound travel at the rear.  I noticed this when I jacked up the rear of the car, and the wheels only dropped (with respect to the body) by about 15mm before they lifted off the ground.  Generally, about 1/3 to 1/2 of the total suspension travel should be into rebound, to allow the wheels to 'drop' into dips in the road.  Given that the shocks should offer at least 75mm of travel, 15mm of rebound travel is a little low, and noticeable when driving the car on bumpy roads in that the rear end can feel as though it is skipping around, especially into dips in the road.

I mentioned this to Jeremy Phillips at the Stoneleigh show in May, and he acknowledged that the Protech shocks I am using seemed to come out slightly shorter (by about 12mm) than the Spax shocks, although they are specced to have the same open and closed lengths.

I then got chatting with the owner of Protech, and he seemed to think that they would be able to change a few bits and pieces around to lengthen my existing shocks at minimal cost.  

So, fast forward to September, and I was due to spend a week in Wiltshire, close to Protech's base, so I gave them a call and arranged to drop my 2 rear shocks in to them (the fronts seem OK, by the way) for modification.

Spring rates

In addition to the lack of rebound travel, I also felt that the Mojo was overly stiffly sprung, especially for mainly road use.  On the track the car is fantastic, but bumpy roads can turn into a roller coaster ride.  The issue seems to be predominantly at the rear, with an uncomfortable bouncing feeling.  In technical terms, the ride frequency is too high.  The ride frequency is a measure of how fast the car would bounce up and down on its springs if you were to remove the dampers.  Generally, road cars are in the 1.1-1.5Hz range, and normally the rear is set to be a slightly higher frequency than the front.

I did some analysis on the Mojo using a spreadsheet I put together and came up with ride frequencies of approx 2.34Hz front and 2.44Hz rear (F-R ratio 1.04).  I then dug out some info on ride frequencies of mid or rear engined sports cars, and found that a Porsche 911 (996) turbo has a front ride frequency of 1.93, rear of 1.59 (F-R ratio 0.83).  This was interesting in that the rear was significantly lower than the front (This tallies with comments often made by road testers about the 911, and the nose 'bobbing' around more than the rear over bumps).

Some reasons for this are:

  • To tame the notoriously lively back end of the 911 (a lower roll stiffness gives less weight transfer at the rear, and hence more at the front, which moves the balance towards understeer)
  • To help with traction on uneven surfaces.

Given that the weight distribution of the Porsche is pretty much identical to the Mojo at 40/60 F/R, I decided that going for a lower ride frequency at the rear could work on the Mojo as well.  Disadvantages are that it will reduce the total roll stiffness (so the Mojo will roll a little more) and that there may be a little more understeer.  I don't expect these to be issues as the Mojo rolls very little at present, and has a propensity to oversteer (sometimes snappily!).

As a first stab, I did the analysis again with 300lb/in rear springs (the original setup was 400lb/in all round) which drops the rear ride frequency to 2.19 (ratio F-R of 0.94), so a reduction of 0.25Hz- surprisingly little!  However, not wishing to make too large a change at first, I have decided to try this setup, retaining the original front springs for the time being.  If I decide to reduce the rear rates significantly more, I would need to also reduce the front rates to maintain a reasonable balance, and then the roll stiffness would really start dropping off.  I could counteract this with anti-roll bars, but things quickly start getting complicated!

I mentioned my plan to Protech, who may be able to dig out some suitable springs for my visit to them- otherwise I'll be ordering some.

It will be interesting to drive the Mojo again after the mods...

Driving Report

Well, my visit to Protech was very successful.  I dropped the rear shocks in on Saturday morning, and Kevin had a dig around in their supply of springs.  All of the 300lb/in springs were hiding, so my choice was 275 or 325, both in 7.5" lengths rather than the original 6" springs.  I decided to try the 325s, not wishing to make too big a change in one go (these give a rear ride frequency of 2.26Hz, ratio of 0.97).

I returned on Wednesday, and Kevin chatted through what they had done:

1. Fitted 8mm longer damper shafts

2. Updated a few internal components to reflect what is 'current' spec

3. Refilled with new oil

The repercussions of point 2 are that the dampers will be stiffer for a given setting- Kevin suggested running maybe 3 or 4 clicks lower than before. (The Protechs are good in this respect in that if you find you are running on the first click and you still find they are too stiff, you can reconfigure the adjuster to give a lower range).

I was charged 30 for the work which I thought was very reasonable, plus the going rate for the springs.

On returning home I eagerly got on with fitting the shocks back on to the car.  At first a mild panic was caused when I realised that the longer springs meant that the lower spring platform was now looking perilously close to the outer CV joint boot- yikes!  However, once all fitted in place I had a good 5mm, which I decided was enough.  I backed the damping adjustment all the way back to the softest setting, dropped the car onto its wheels and had a quick fiddle with the spring platforms to get the ride height about right (in theory I should now have to go through the corner-weighting procedure once again.  I'll do this at the earliest opportunity).

Then it was time to go for a drive- and what a difference it had made!  The rear of the car now feels much better matched to the front, with a firm but flat, well damped ride with none of the previous high frequency bouncing motion.  I did try adding one extra click to the rear dampers, but have now reverted back to the first click.

Overall, the ride is significantly better.  Some bumps on my journey to and from work that used to really unsettle the back end are now dealt with much more smoothly.  The rear feels well planted over dips and crests, which gives a much more confident feeling.  As a bonus, I've not really noticed the extra body roll that must be now present.  I'm very pleased with this upgrade, and now wish I had done it months ago.  If anything I think I could now reduce the front rates very slightly, but maybe only down to 375lb/in.  For the time being, I'm not going to worry.

February 2006 Update:  As with anything, after a few months I have forgotten just how firm the Mojo felt with 400lb/in springs on the rear, and feel that the 325lb/in rear springs are still a little firmer than ideal.  I have therefore been considering dropping the spring rates further still, however to do this I will need to reduce both the front and the rear to keep a good balance.  The obvious thing to try first is fitting the 325lb/in springs (currently on the rear of the car) to the front, and buying some slightly lower rated springs for the rear.  Using the corner weights and ride frequencies calculator, I decided to try 275lb/in springs on the rear, as this gives a similar front to rear ride ratio (0.99) as the 400-325 setup (0.97).  Given that this is really very experimental, I decided to try and locate some secondhand springs, and a quick search on ebay found exactly what I wanted (amazingly!)- 6.5" long, 2.25" diameter 275lb/in springs!  I stuck in a bid and won the pair for the princely sum of 8.50, bargain!

My only slight issue is whether the 7.5" 325lb/in springs will fit on the slightly shorter front shocks (currently running 6.5" springs) but I think they might just squeeze on.  I will wait until the 275s arrive until I do some checking.

Once the 275 springs arrived, I took off the right hand shocks to try the rear springs on the front.  Unfortunately, they are too long to fit, so I will have to try and source some 6.5" 325lb/in springs (although anything of that length in the 300-350lb range would be OK).

I decided to fit the 275s to the rear anyway, which will give a front to rear ride ratio of 0.9, still a fair bit less extreme than the Porsche setups discussed earlier.  A quick drive around the local backroads and it felt fine, some more miles are required to give a final verdict though...

Whilst I had the shocks off, I also wanted to try the Protech instructions for lowering the range of damping adjustment on all 4 shocks, as I am running very much at the bottom of the range at the moment (ie first click all round).  This worked fine on 3 of the shocks, but on one of the rears I could not get the grub screw to shift, even after copious soaking in oil.  I will give Protech a call to see if they have any suggestions.  For the time being I set both rear dampers back to the same setting!

I also noticed whilst visiting the Protech website that they are now doing double adjustable shocks, ie different adjustments for both bump and rebound.  That could be very useful...!

April 2008 Update:  As ever, I was looking to improve the Mojo further still, so I hatched a plan to soften the front springs significantly, but to retain the roll stiffness and stiffness distribution by investigating the fitment of a front anti-roll bar (ARB).  I confirmed that a bar could feasibly be routed between the front of the chassis and the radiator, and that a droplink could be located onto the lower damper bolt.  I then used my setup spreadsheet to come up with a setup.  I settled on 275lb/in springs and a roll bar around 12mm.  

However, to start with I decided to just try the spring change alone.  I spotted a pair of practically new springs for sale on the Westfield boardroom, so got a pair of springs for 20.  Even better - they were powdercoated red!

I swapped the springs and was amazed at the improvement.  The pitching motion was improved and ride quality far better.  And better still, I didn't really notice the reduced roll stiffness, and the car feels no more twitchy despite the roll stiffness distribution moving back towards the rear end.  Admittedly I've not used the car on track with this setup so haven't explored the limits of the handling, but for road use I have put the ARB upgrade on hold!

To bring you up to date on the Protech damping adjustment mentioned previously, I ended up taking the dampers back to Protech anyway as one of the rears developed a leak that was picked up at the MOT.  Protech inspected the damper and found a gouge in the damper rod that was causing the leak.  The other damper was also displaying some odd behaviour on their test rig, so both were stripped down, rebuilt and revalved with their softest valve at my request.  All at no charge!  Protech come highly recommended for after sales service!